1. Field Of The Invention
The present invention relates to the fabrication of corrugated board. Specifically, the present invention relates to a method and apparatus for bonding a polymer plastic coated paper liner medium to a corrugated paper medium web.
2. Background Of The Invention
Over many years of use, corrugated paper board has proven to be extremely valuable in many, various uses due to the low cost and high strength to weight ratio. However, conventional paper board has also been limited to applications where the structural product fabricated therefrom will be protected from the invasion of moisture. This is due to the dramatic loss of strength caused by moisture absorption.
Efforts in the past to protect paper-base corrugated board from moisture invasions have included both wax and plastic coating of either the constituent webs prior to board fabrication or of a product assembled therefrom. Such past efforts have been less than satisfactory from several points of view.
As a water-proofing agent, wax has proven to lack the necessary strength and toughness. When applied to corrugated containers, handling abuses invariably cut or crack the wax coating at some point. If moisture is present, penetration will occur at the crack.
Great success has been achieved with polymer plastic as a moisture-proof cladding of a substrate paper web as described in U.S. Pat. Nos. 3,849,224 to H. L. Hintz et al. and 3,799,837 to O. Witnes et al.
Difficulties with commercial production techniques, however, have plagued the development of poly-coated paper as a corrugated board constituent.
Simultaneous with the development of water-proofing techniques for paper substrates, competitive manufacturing techniques for solid plastic web corrugated board have been developed. Although, the strength of solid plastic corrugated board is generally comparable to paper base board and completely unaffected by moisture, the relative cost is high and strength diminishes at higher ambient temperatures. In general, therefore, as a competitive alternative for most high moisture uses, solid plastic corrugated board presents a cost ceiling for plastic coated paper corrugated board.
The predominance of production problems for plastic coated paper corrugated board center about the single facing operation where a medium web is first formed into a corrugated continuum between the meshing nip of two corrugating rolls. Immediately thereafter while the corrugated medium is still held contiguous with the undulating surface of one of the corrugating roll pair, a liner web is bonded to the flute crests of the medium. This bonding occurs within 90.degree. to 180.degree. around the corrugating roll from the corrugating nip. Within an additional circumferential traverse of 90.degree. to 180.degree. about the corrugating roll, the resulting single-face board is stripped from the corrugating roll.
At high production rates and machine speeds, the time interim between joinder and stripping may be less than one second. This time interim is too short for the setting of most adhesives that are compatible with the plastic web coating. Accordingly, if either, the liner or the medium is plastic coated, a quick setting, hot melt adhesive must be used. However, hot melt adhesives are relatively very expensive for this application and, if used, the cost advantage of paper base board over solid plastic corrugated board is substantially lost.
As an alternative technique for bonding a liner web to a corrugated medium web on a single facer, the adjacent plastic coat of either web may be heat plasticized to a tacky consistency and then fused to the other web in the single facing nip. It is this technique to which the disclosure of U.S. Pat No. 3,811,987 to Wilkinson et al. is addressed. The Wilkinson et al patent uses an open flame to heat the adjacent coat of the liner web immediately prior to the single facing nip.
Experience with the Wilkinson et al. technique has been disappointing first, from the perspective of bond uniformity due to the non-uniform distribution of heat across the web from the burner head and, secondly, due to consequent pin holing in over-heated areas which destroys the moisture proof objective.
Although Wilkinson et al. were aware that the bond surface of the plastic coated liner could be sufficiently heated for plasticizing over a conventional, pre-heating drum, the technique was discounted by them because of web sticking to the drum surface. However, A.L. James in his U.S. Pat. Nos. 3,360,412 and 3,457,139 and his Dry Lamination article published in the July, 1970 issue of Paper, Film and Foil Converter, pages 29-31 taught that polymer plastic film could be plasticized over heated drums that were surface coated with a high temperature, non-sticking polytetrafluoroethylene resin plastic such as Teflon.
With these tools, the problems of fusing a board liner to the corrugated medium flute crests are largely reduced to heat transfer management coordinated with speed control.
In the past, uncoated liner and medium webs have been preheated over heating drums to a regulated temperature by controlling the degree of wrap that the web is allowed to traverse over the drum circumference. U.S. Pat. No. 3,004,880 to K.B. Lord teaches a technique whereby web temperature sensors control the angular position of automatically adjustable wrap control rollers. By this technique, the machine speed is set independently by the operator and the temperature regulating mechanism adjusts the degree of web wrap about the heating drum accordingly.
Because of the relatively narrow, tolerable "dead band" temperature for a plastic coated bonding surface, the Lord technique of temperature control tends to instability due to a continuous searching or hunting of the wrap control rollers for a correct setting under only moderately variable conditions.
Additionally, if a heater drum technique is used for plasticizing poly-coated paper at high production speeds, a relatively large traverse surface length is sometimes required to develop the necessary temperatures. One natural consequence of a large traverse surface length is a large diameter heater roll which is not only expensive to acquire but expensive to heat. Some advantage may be gained, however, by using a majority of the drum circumference as the traverse surface arc. However, this expedient is encumbered by the fact that the web supply point is fixed at a usually close proximity to the heating drum and when the heat transfer arc exceeds 180.degree., the web course will interfere with the heater drum structure.